1. Field of the Invention
The present invention relates to a device for separating fuel components.
2. Description of the Related Art
For example, in an engine having a high compression ratio, high-octane fuel is needed to restrain knocking. However, in low engine load operations in which knocking rarely occurs, the engine may not be required to use high-octane fuel, but low-octane fuel. Accordingly, it is suggested that the engine not use high-octane fuel (high-octane gasoline) which is expensive, but regular gasoline which is separated into high-octane fuel components and the remaining fuel as low-octane fuel by a device for separating fuel components.
The device for separating fuel components separates aromatic components as high-octane fuel components from fuel by means of a separating membrane and makes the remaining fuel low-octane fuel. In such a device for separating fuel components, un-separated fuel is preferably heated to improve a separating ability of the separating membrane. Therefore, the low-octane fuel remaining when high-octane fuel components are separated from fuel is maintained at a high temperature and may be required to be cooled down before being stored in a low-octane fuel tank. Accordingly, it is suggested that a heat exchanger between the un-separated fuel and the separated low-octane fuel be provided and the separated low-octane fuel be cooled down by the heat exchanger (for example, refer to Japanese Unexamined Patent Publication Nos. 2004-232624 and 2008-095666).
However, a part of the low-octane fuel remaining when high-octane fuel components are separated changes from a liquid phase to a gas phase because the un-separated fuel is heated. Thus, a flow resistance of the low-octane fuel in the heat exchanger is large and a heat transfer coefficient of gas phase is smaller than that of a liquid phase. Therefore, to sufficiently cool down the low-octane fuel including the gas phase, a very large heat exchanger is required, and it is thus difficult for the device for separating fuel components with such a very large heat exchanger to be mounted on a vehicle.